tests/drm-writeback-screenshot-test: Grab the first available output

This is basically a small follow-up from commit 'drm-smoke-test: Explicitly set-up the output name DRM test' which did the same thing.

With multiple virtual outputs in the system, we would override the output and pick/use up the last one available, with the client placing the window/surface on the first output. This would make things easier when testing and transitioning to multiple virtual outputs.

Signed-off-by: Marius Vlad marius.vlad@collabora.com